The effect of tillage on the composition and structure of the tilled soil layer

How ploughing changes the structure of the topsoil

By regulating soil density and porosity, an agronomist directly influences plant development and yield. The ratio of the solid phase of the soil to various types of pores determines how water and air will circulate in the topsoil. Modern implements allow these parameters to be adjusted according to the requirements of a specific crop and field conditions.

In the chernozems of the Kuban region, four main physical states of the topsoil are distinguished. Each corresponds to strictly defined indicators of density and porosity. These data help assess the degree of field compaction before selecting a tillage technology.

State of the topsoil	Density, g/cm³	Total porosity, %	Ratio of capillary to non-capillary pores, %
Very loose	Less than 0.95	More than 64	Approximately 55 : 45
Loose	0.95–1.1	58–64	Approximately 60–65 : 35–40
Dense	1.25–1.36	49–52	Approximately 75–80 : 20–25
Highly compacted	More than 1.35–1.45	Less than 47–48	Share of non-capillary pores decreases to 5–10

Conventional mouldboard ploughing radically changes the structure of the topsoil. It sharply increases total soil porosity and reduces its density. At the same time, the share of capillary pores also decreases, which is important to consider when planning sowing.

• Soil density after ploughing — 0.8–0.9 g/cm³
• Total porosity — 60–65% and more
• Capillary porosity — 50–55% of pore volume

Such loosening is especially important for leached and slaked chernozems, heavy meadow, dark-gray and gray forest soils, as well as compacted chernozems in depressions. On these soil types in the southern foothill zone of the Krasnodar Krai, ploughing serves as the primary means of reducing excessive capillary porosity. However, the quality of crumbling depends directly on the initial state of the field.

The quality of soil block crumbling depends directly on soil moisture during operation. When working on an over-dried field with a density of about 1.3 g/cm³ and moisture at the wilting point (WP), cloddiness increases sharply. At the same time, increasing the ploughing depth on chernozems improves the water stability of the top layer, as structured soil from subsoil horizons is brought to the surface.

Features of conservation and surface tillage

Using sweep cultivators for primary tillage crumbles the soil less than a plough. By the beginning of spring field work, minimum tillage leaves the field denser than classical autumn ploughing. The magnitude of this difference depends on the type of soil being processed.

Soil type	Density difference compared to autumn ploughing, g/cm³
General indicators	0.1–0.15 (soil is denser with conservation tillage)
Heavy soils of the southern foothill zone	Close to untreated areas

Sweep cultivators work at a shallow depth — up to 12–14 cm — and practically do not invert the furrow slice. In arid conditions, this allows protecting the loosened layer from rapid moisture loss. At the same time, crop residues are preserved, which protect the soil from wind erosion.

Universal sweep and ripper tines crumble the earth significantly better than shallow cultivators. After their passage, the porosity of the treated layer exceeds 65%, and density decreases to 0.8–0.9 g/cm³. The share of non-capillary pores increases to 40–50% of total porosity.

How discing, packing, and milling change the topsoil

Disc harrows and cultivators loosen only the upper part of the topsoil. Proper selection of implements allows for improving the physical properties of this layer. However, on wet soils, the mechanical action of discs can lead to the opposite effect.

Packing mainly affects the upper half of the topsoil. Soil density after roller passage depends on its mass, initial humidity, and field structure. If soil moisture exceeds 20–22% of absolutely dry mass, compaction will be noticeable. Conversely, in dry soil (at wilting point or lower) or in the presence of large clods, rollers practically do not increase density.

On heavy clay soils, the cohesion of dry aggregates is very high, so rolling does little to change their structure. Compacted chernozems, heavy grey forest and meadow soils with humidity near the wilting point hardly react to the passage of rollers. The nature of changes in leached chernozems under the influence of different types of rollers is shown in the table.

Roller model	Specific pressure, g/cm²	Soil humidity and aggregate content	Compaction depth, cm	Change in density and porosity
ZKK-6A	200–250	About 22% humidity, clods >10 mm make up 38% of the mass of the arable layer	8–10	Density of the upper layer increases noticeably
ZKVG-1.4	300	Optimal soil humidity	up to 12	Density of the upper layer increases noticeably
ZKVG-1.4	300	About 16% humidity (wilting point)	0–5	Total porosity in the 0–5 cm layer decreases by no more than 2–4%, density changes little

Using a rotary tiller on leached and ordinary chernozems, as well as on meadow soils, significantly loosens the cultivated layer. Milling reduces the number of clods — for example, on leached chernozem the mass of the cloddy fraction is reduced by 20% or more compared to ploughing. At the same time, the risk of grinding the soil into dust depends on its type. On leached chernozem, even at low humidity, the amount of dust does not increase, whereas on ordinary chernozem in a dry state, milling sharply increases the proportion of the dusty fraction.

• Soil density after disking — 0.8–0.9 g/cm³
• Soil porosity after disking — 60–65 %
• Soil porosity after milling — over 65 %
• Proportion of non-capillary porosity during milling — 40–45 %

How long does the effect of tillage last

The period during which the soil retains its imparted structure depends on its mechanical composition and climatic conditions. Heavy soils with a high content of physical clay and silt are quickly compacted by heavy rains. Conversely, if soil aggregates possess high water stability and a coarse-cloddy structure, the arable land resists capping and settling longer. Intense downpours nullify the results of tillage the fastest.

The structure of the arable layer lasts longest after ploughing. On ordinary and typical chernozems of the steppe zone, the difference in density and hardness between ploughed and unploughed areas is clearly traceable for two years. In wetter conditions, this period is significantly reduced.

In the southern piedmont zone, the arable structure is destroyed much faster due to the heavy mechanical composition of the soils and abundant precipitation. During the moisture accumulation period, more than 220–250 mm of precipitation falls here, and the content of physical clay in the soil reaches 75–80%. The situation is aggravated by the low water stability of waterlogged piedmont soils, which on compacted chernozems in spring is only 25–36%.

Indicative data were obtained during monitoring of the density of compacted chernozems in experiments. Already in the spring of the year following autumn ploughing, the soil density returns to the levels of areas left unworked since autumn. The difference between ploughed and unploughed backgrounds does not exceed 0.05–0.07 g/cm³, and in wet years, it is completely leveled out.

Effect of precipitation and machinery passes on arable land density

The effect of cultivation and tillage with disk implements does not last long — only a few ten-day periods. A single heavy rainfall with an intensity of more than 20–30 mm can completely nullify the results of the work. On heavy soils, after such rains, the difference in density between worked and undisturbed areas practically disappears. Tooth harrows provide an even shorter-lived effect, which lasts only until the first rain of 15 mm strength.

Heavy machinery compacts the arable layer all the more strongly the higher its humidity and the heavier its particle-size distribution. On chernozems, the compacting effect of wheels increases sharply at 24–25% humidity. In the southern piedmont zone, where clay soils with a physical clay content of more than 70–80% prevail, spring overcompaction is especially great. Beneath a thin, dried-out crust, a waterlogged layer often persists for a long time, which is easily subject to deformation.

Tracked vehicles have a significantly gentler impact on the soil. In the track of a crawler tractor, soil density increases by only 0.15 g/cm³ compared to an untreated area. In ordinary chernozems containing less physical clay, spring waterlogging is less pronounced, which is why the damage caused by wheeled machinery is lower there.

Depth, cm	Soil density May 16, g/cm³		Soil density August 29, g/cm³
	Outside track	In track	Outside track	In track
0–5	0.98	1.41	1.10	1.19
5–10	0.16	1.44	1.21	1.38
10–15	1.27	1.46	1.36	1.43
15–20	1.25	1.45	1.35	1.44
20–25	1.26	1.40	1.37	1.39
25–30	1.29	1.34	1.29	1.35

Physical maturity of soil and tilth quality

The quality of ploughed field fragmentation during tillage depends directly on its humidity. The soil crumbles best into structural elements in a state of physical maturity. At this point, the water films around soil particles no longer allow them to stick together, but there is not yet enough moisture for the soil to become plastic and start smearing.

For chernozems, the maturity interval ranges from the transition of film-meniscus moisture to capillary moisture up to a level slightly below the field moisture capacity. Above this limit, stickiness and fluidity increase sharply. Physical maturity is usually associated with the capillary rupture moisture, which for leached chernozem is 23.6–25.5%.

• Physical clay in foothill soils — more than 70–80%
• Critical humidity for compaction of chernozems — 24–25%
• Maturity window of heavy soils in spring — 5–10 days
• Capillary rupture moisture of leached chernozem — 23.6–25.5%

The medium-compacted arable layer of leached and ordinary chernozems crumbles well at a gravimetric humidity of 22 to 27%. On heavy soils, this window is much narrower: on compacted (slitozem) chernozem, maturity occurs only at a humidity of 26–28%. Because of this, the heavy soils of the southern foothill zone and meadow soils remain mature for a very short time — in spring, this period lasts only 5–10 days. If they are cultivated before this date, the soil is cut into large wet clods, which become ultra-hard after drying.

The heavy soils of the southern foothill zone reach maturity for tillage much later than the ordinary chernozems of northern regions. Due to their high clay content, they release moisture slowly in the spring. Under the upper dried layer 4–6 cm thick, a waterlogged stratum can persist for 10 to 20 days. Its desiccation accelerates only after the appearance of deep cracks.

How tillage speed, frost, and humidity affect the structure of the ploughed field

Increasing the speed of implements allows for earlier commencement of spring field operations. High-speed tillage expands the range of physical soil maturity, thanks to which layers with high humidity crumble significantly better. For example, by increasing the plough speed from 1.06 to 1.46 m/s on chernozems, the soil crumbles well even at a humidity 15–20% above the optimum. At a speed of 1.46 m/s, the fragmentation quality remains satisfactory even at the lower limit of soil plasticity.

No tillage implement can match the natural effect of frost in terms of the quality of field preparation. If the second half of winter was cold and frosty, the soil acquires an ideal fine-grained structure after spring tillage. In conditions of a rainy winter with weak freezing, spring tillage inevitably produces a cloddier aggregate composition. Frost most effectively "improves" the structure of moderately moist and loose arable land.

In dense and waterlogged soil, water is held in fine pores by sorption forces and freezes at lower temperatures. When all pores are filled with water, the expansion of ice does not loosen the soil but merely increases cloddiness during spring tillage. Such a phenomenon is characteristic of waterlogged foothill soils. The main condition for high-quality spring preparation of such areas is the prior elimination of winter-spring moisture stagnation.

Any mechanical impact on the ploughed field leads to the destruction of the structure and the formation of dust. The degree of pulverization depends on the soil humidity at the time of machinery passage and its mechanical composition. Dry soil with low clay content is most vulnerable to mechanical grinding by working tools.

• Increase of plough speed for moist soil — from 1.06 to 1.46 m/s
• Allowable humidity excess during high-speed ploughing — by 15–20%
• Dust limit in dry leached chernozem after 10 tillage passes — 9–10%
• Dust limit in dry ordinary chernozem during multiple tillage passes — up to 12–18%

Soil type	Condition and moisture	Intensity and type of tillage	Dust content after tillage
Loamy chernozem	Ready-to-till state	Intensive tillage	Does not increase (remains at the initial level)
Leached chernozem	Ready-to-till state (moisture 24.8 %)	Multiple tillage	No more than 4–5 %
Leached chernozem	Dry state	10-fold (cultivator, harrow, stubble breaker, roller)	Increases from 4–5 % to 9–10 %
Ordinary chernozem	Ready-to-till state	Multiple tillage	Practically does not increase
Ordinary chernozem	Dry state	Multiple tillage	Increases up to 12–15 % (in some cases up to 18 %)